The definitions described hereinbelow underlie the present invention. “Adsorptive material separation” is understood to mean the removal of one or more components from a fluid phase by selective adsorption of said component(s) to a solid phase, the “adsorbent” (plural “adsorbents”). The field of the invention generally relates to material separation in liquids, liquid being referred to hereinafter as “medium”. Adsorbents are porous solids which can selectively form bonds with certain components of fluids via functional surface groups referred to as “ligands”. In addition to the “particulate adsorbents”, also referred to as chromatography gels, which have been known for a long time, further “nonparticulate adsorbents” based on a completely different matrix have become established. These are so-called monolithic adsorbents composed of a three-dimensional porous solid body or support based on microporous membranes composed of various polymers. Adsorption membranes refer to planar adsorbents having pores passing through from one side to the other.
According to the invention, target substance(s) and/or contaminant(s) are referred to as “adsorbate” and used in the singular, though two or more different substances may also be involved. The “capacity” of an adsorbent is understood to be a quantitative measure of its uptake capacity for adsorbate. The capacity is based on a defined amount of adsorbent.
The present invention relates to the technical field of depth filter sheets and depth filter sheet systems comprising inorganic layered double hydroxides, as described in DE 10 2008 037 678 A1 for example, which are used for contaminant removal in biotechnological processes. In this connection, said depth filter sheets and depth filter sheet systems comprising inorganic layered double hydroxides are used to remove undesired biological components, such as, for example, nucleic acids, cellular proteins, viruses or endotoxins, from process solutions as filtration medium, the undesired biological components being adsorptively bound to the depth filter, and the desired target products, such as, for example, antibodies, hormones, enzymes, (poly)peptides and, more particularly, therapeutic proteins, however, being able to pass through the depth filter unimpeded.
Nucleic acids are to be understood here to mean all naturally occurring nucleic acids such as ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), more particularly DNA of genomic or else epigenomic origin from eukaryotic, prokaryotic and viral sources, and very particularly nucleic acids having a chain length of more than 15 base pairs.
Said depth filter sheets and depth filter sheet systems comprising inorganic layered double hydroxides have a high capacity for depleting DNA and other contaminants and have, at the same time, a low amount of extractable constituents from the filter material, and so they are suitable for use before and after the chromatographic process step in downstream processes. The breakthrough of contaminants is a critical factor in validated biopharmaceutical processes. The use of depth filters and depth filter systems for contaminant removal requires a way of checking the quality of the filter and of being able to make a prediction about the binding capacity and the integrity of the depth filters and depth filter systems.
Because of the new types of properties of depth filter sheets and depth filter sheet systems and because of the associated new fields of use for contaminant removal in downstream processes, it is necessary to test the depth filter sheets and depth filter systems comprising inorganic layered double hydroxide before their use with respect to their integrity and functionality. The further development of depth filter sheets for critical applications also requires the further development of ways of testing said new properties.
To date, conventional depth filters and depth filter systems are used in biopharmaceutical processes to remove particles, especially in the clean-up of cell culture solutions to remove cells and cellular constituents. In this connection, a qualification of the filters is done by checking the filter thicknesses and water flow rates. An additional integrity check is not necessary for these noncritical applications.
JP 3233357 B2 discloses an integrity test for depth filters, in which said filter is subjected to a diffusion test at 0.2 bar. Using said integrity test, it is possible to establish only very large defects above a “bubble point” value of 0.2 bar. In the integrity test known from JP 3233357 B2, there is also no correlation between the result of the integrity test and the binding capacity of the depth filter for biomolecules.
JP 3371366 B2 discloses an integrity test method similar to that of the aforementioned document, in which depth filters wetted with water are subjected to a gas permeability test near the “bubble point” value.
For contaminant removal, use is also made of nonparticulate adsorbers, such as, for example, the membrane adsorber Sartobind® Q from Sartorius Stedim Biotech GmbH. For the qualification of said nonparticulate adsorbers, the applicant discloses in DE 10 2010 004 188 A1 a method in which the nonparticulate ion-exchanger adsorber a) has alkaline solution applied thereto in the case of an anion exchanger or has acid applied thereto in the case of a cation exchanger, b) is rinsed with water, c) has a liquid ion-containing medium applied thereto with detection of the concentration of the broken-through ions by means of an ion-sensitive probe and d) the concentration profile detected in c) is compared with that of a nonparticulate ion-exchanger adsorber of known integrity.
In DE 10 2010 004 190 A1, the applicant discloses a further method, in which a nonparticulate adsorber (membrane adsorber) loaded with an adsorbate under conditions under which the adsorbate is retained by the nonparticulate adsorber (membrane adsorber), the breakthrough of the adsorbate, for example of phosphate ions, is detected with very high accuracy by means of a secondary reaction, and the breakthrough characteristics of the phosphate ions is compared with those of a nonparticulate adsorber of known integrity.
The basis of the two methods disclosed by the applicant is that, during the application of a liquid, ion-containing medium to the membrane adsorber, these ions are completely retained until the breakthrough volume is reached and the excess ions in the filtrate are appropriately detected only after the breakthrough volume is reached. In this connection, the binding of the ions to the matrix of the membrane adsorber is done to the porous structure. The basis is a microfiltration membrane comprising its multiplicity of pores having a relatively narrow pore size distribution with a minimum and a maximum pore size. Functional groups are chemically bonded to the surface of said pores. As a result of the interaction with said functional groups, the adsorbates are adsorptively bound when flowing through the membrane.
These methods disclosed by the applicant are generally not suitable for depth filter sheets and depth filter sheet systems, since a breakthrough of the liquid ion-containing medium takes place immediately, and this is to be expected in view of the typical structure of a depth filter. Depth filter sheets have a structure completely different to that of membrane adsorbers. They consist of a sheet of cellulose fibers lying on top of one another in an irregular manner and pulverulent adsorbents bound to the cellulose fibers by means of a binder. Depth filters have a broad pore distribution without the exclusion of a minimum and a maximum pore size. As a result, depth filters generally do not have an absolute separation limit. For a depth filter, the pore shape and distribution is irregular and not clearly defined owing to the fiber structure. Therefore, in a depth filter, the density and distribution of the adsorbing particles and also their binding sites are more irregular than in the case of a membrane adsorber.
When ion-containing media are applied to a depth filter, these ions are generally depleted, but not completely retained. For instance, the application of ions according to DE 10 2010 004 188 A1 or DE 10 2010 004 190 A1 to conventional, commercially available depth filter sheets and depth filter sheet systems containing kieselguhr as pulverulent adsorbents, such as, for example, S9P from Sartorius Stedim Biotech GmbH or Beco Steril S100 from Begerow, shows in said application an immediate breakthrough of the described ions and adsorbates. Consequently, these integrity test methods described by the applicant for the qualification of nonparticulate adsorbents are not suitable for determining the integrity and functionality of depth filter sheets and depth filter sheet systems, i.e. checking the quality, more particularly the integrity, of the filter and making a prediction about binding capacity.